Objective The objective of this experiment is to observe the apparent rotation of the sun by tracking sunspots across its surface and to predict the period of rotation of the sun.

Introduction Before Galileo invented the telescope it was thought that the earth was the center of the solar system and the sun (and all the planets) orbited around earth. Once we started making observations into space we adopted our current belief of the sun being the center of the solar system and the planets orbiting around the sun. By means of tests and observations through advanced telescopes this theory has held true. However, once we adopted our current belief, it was still unknown whether or not the sun rotated. We predict that the sun rotates because all other bodies of mass we encounter (planets, satellites, asteroids) rotate so it isn’t a surprise that the sun, a star, would rotate as well. Galileo tested this theory by looking at imperfections on the surface of the sun he called sunspots. Sunspots are concentrated zones of magnetic activity causing that spot of sun to cool relative to the rest of the surface creating a dark oblong spot that can be seen from earth (Grigor’ev, Ermakova & Khlystova, 2012). According to the study done by Grigor’ev, Ermakova & Khlystova, 100 years ago Mount Wilson Observatory concluded that sunspots have magnetic fields and they start to form as a buoyant loop-like field in the photosphere. Then, a number of days later the loops finish and the sunspot is formed. They determined in their study that sunspots have a complex multilayer structure of developing arches. These arches form “moat” structures and arches of different magnetic fluxes in the active region of the sunspot (pp 878- 881). By tracking these spots we are able to determine how fast the sun rotates and how many days it takes to make a full rotation. A major assumption we are making when observing these sunspots are that the dark sections we see on the sun actually belong to the sun and aren’t just objects floating in space. Another assumption we make is when we are calculating the angular velocity of the sun we assume these spots don’t move around or change shape significantly on the suns surface. If they moved it would create uncertainties in our calculations because we rely on the angles we create from our pictures and if the spots are moving around on the sun it would make it difficult to predict how fast the sun is rotating.

Equipment
Translucent Paper
Protractor
Ruler
Pencil
Pictures of the sun

Procedure First we determined that there are sunspots on the sun by looking into a solar telescope. After that, we went back to the lab and looked at a sequence of pictures of the sun during a period of eight days (September 20, 2013- September 27, 2013) Next, we located one sunspot that appeared on every image of the sun and we traced that sunspot on a translucent piece of paper for eight days as it moved across the sun. Each day the sunspot appeared in a slightly different spot. We connect all the spots with a line and made a 3D effect by attaching a piece of paper to that latitudinal line. This represented a cross sectional view of the sun. With the radius of our latitudinal line we drew a semicircle. We then transferred the sunsposts we previously found on to the cross sectional area and found the angles between each sunspot. With these angles we estimated the number of days it takes for the sun to complete a rotation.

Table
Picture
Angle
ΔT
T
1
13
1
27.7
2
14
1
25.7
3
14
1
25.7
4
13
1
27.7
5
15
1
24
6
14
1
25.7
7
11
1
32.7
Latitudinal Line: 16cm
Radius: 8cm

Calculations
T= ΔT x 360° Average Day = The sum of (Angle 1-7) = 27.03 Θ 7 Example: Day 1
T= 1 x 360 = 27.7 13

Results It takes 27.03 days for the sun to complete one rotation.

Questions
1. 27.69-25.71 = 0.99 days 2
The uncertainty in our calculations is about